He needs exercise and mental stimulation more than anything else. Walks, especially if it's after work and you're tired, and more likely to be mental stimulation than exercise. Do you have an area you can play fetch in? Or maybe you can try biking or urban mushing? Anything where he's running is going to be a lot more effective than a walk even if it's long. Some people also teach their dogs to use a treadmill, and swimming is fantastic exercise if you have a place he can go!
As for mental stimulation, training is excellent! You can also give him stuffed kongs and puzzle toys. Personally I would leave him in his crate while you're gone until he is reliable, but some people also teach their dogs to find treats that they've hidden around the house. Socialization, meeting new people, new dogs, sniffing new things, and encountering new situations is also great mental stimulation.
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